Integrated circuit (IC) devices and other electronic components are normally enclosed in a protective package to prevent physical damage to the device from handling, from process chemicals, or environmental exposure. The physical package also provides a means of electrical connection from the IC device to a set of pins, leads, or contacts of some sort. The pins, leads, or contacts are typically used to connect the IC device to a printed circuit board (PCB) assembly and other packaged IC devices. There are many different types of packages used for IC devices including dual inline packages (DIP), small outline packages (SOP), pin grid array packages (PGA), ball grid array packages (BGA), and land grid array packages (LGA). The type of package that is used depends on the number of I/O connections on the device as well as the product application.
Portable product applications such as cellular telephones, camcorders, and laptop computers require IC packages that are very small to keep the size and weight of the product to a minimum.
Chip scale packages (CSP) are a classification of IC package that are only slightly larger than the size of the bare die. The typical chip scale package footprint size ranges from 1 to 1.2 times the size of the bare die footprint. The chip scale package provides an advantage over other IC packages where size, weight, performance, and, cost are extremely critical. Chip scale packages are very similar to ball grid array packages (BGA), only smaller. Both CSP and BGA packages use solder balls or solder coated bumps to provide a connection to the PCB. The pitch between solder ball connections on a BGA package is typically between 1 to 2.5 mm while the pitch between connections on a chip scale package is typically between 0.5 to 1 mm.
The terminal contact pads on an IC device can be located along the periphery of the die or in an area array configuration. In order to keep the die size as small as possible, contacts located along the periphery of the die are placed as closely together as possible. The minimum spacing between peripheral contacts (0.08 to 0.1 mm) is determined by the physical limitations of the equipment used to bond wires or other connections to the contact pads. The minimum spacing between area array contacts (0.2 to 0.4 mm) is determined by the die size and the number of contact pads required. Since the minimum line spacing on a typical PCB is limited to 0.2 mm and the minimum via size is limited to 0.3 mm, it is difficult to attach an IC device directly to a PCB without some means of fanning out or redistribution of the contacts on the IC device to a wider spaced array of contacts. The chip scale package is designed to provide the redistribution from the closely spaced contacts on the IC device to a wider array of contacts as well as provide a means of controlling the stresses due to thermal expansion mismatch between the silicon die and the epoxy glass PCB.
The thermal coefficient of expansion (TCE) for the printed circuit board is much higher than the TCE of the IC device. The mismatch between the TCE of the printed circuit card and the IC device would cause the connections to break after repeated thermal cycling caused by the power on and off of the device under normal application conditions. In order to avoid this problem, it is necessary to provide a compliant interface between the IC device and the PCB or encapsulate the solder connections in a semi-rigid material to absorb or dissipate the cyclic stresses.
Thermal performance of the package is critical to high density and high performance IC devices. The smaller package size and footprint of the chip scale package limits the surface area to dissipate heat by convection. In order to keep the operating temperature of the IC device under control, it is often necessary to use a heat sink to increase the surface area and spread the heat away from the device more efficiently.
Although a majority of CSP devices will be permanently attached to a PCB, it is desirable for certain applications to be able to add or replace an IC device for upgrading the function of a product. While many chip scale packages use a compliant interface between the IC device and the connection to the PCB, the interface is not designed to be a pluggable connection. The ideal requirements for a pluggable interconnection include a compliant or spring conductor with Au or Pd/Ni contact surfaces, a mating action that creates a wiping contact motion, and a redundant contact geometry.
There are a number of commercially available connectors for BGA and LGA packages. These connectors are separate components from the IC package and use a variety of contact geometries and materials. For a pluggable chip scale package, the contact geometry must be proportionately smaller than a similar BGA or LGA connector. Conventional metal spring contacts are difficult to form for connections closer than 25 mils. Elastomeric connectors can be made for contact pitches in the range of 10 to 25 mils. Uneven spacing and high resistance of the contact elements are a limiting factor for most elastomeric connectors.
Alignment of a BGA or LGA package to the connector and PCB becomes more challenging as the contact pitch gets smaller. The alignment tolerance for a pluggable CSP would be less than .+-.1 mil for a contact pitch of 20 mils. Although TCE is not a concern for solder joint fatigue life in a pluggable chip scale package, excessive TCE mismatch between the CSP contacts and the PCB could cause misalignment of the contact on a large IC device with close spaced contacts.
A clamping fixture is an additional requirement for connecting an LGA package to a PCB. The clamping fixture is designed to press the LGA package against the interposer connector and the PCB. The clamping fixture compresses the contact elements on the interposer connector to create the required normal force for a reliable connection. The clamping fixture typically provides a means of aligning the LGA package to the interposer connector and the contacts on the PCB. A pluggable chip scale package would have similar alignment and clamping requirements to an LGA package.
Chip scale packages includes chip on board, flip chip, micro BGA, SLICC, and OMPACS. Chip on board (COB) is a packaging technique that is used to bond a bare IC device to a PCB with the contact pads facing away from the surface of the PCB and uses conventional wire bonded connections from the IC contact pads to the pads on the PCB. The COB package is encapsulated in an epoxy material for protection from mechanical and chemical damage. Flip chip or C4 packaging uses solder balls on the IC contact pads for direct connection to the PCB or other wiring substrate. Micro BGA packaging uses a polyimide film circuit with solder balls for connection to the PCB and compliant leads connecting the IC device to the polyimide film circuit along with a layer of compliant material separating the IC device from the polyimide film circuit. The slightly larger than IC carrier (SLICC) uses flip chip connections for attachment to an FR4 carrier with redistribution wiring and solder balls for connection from the FR4 carrier to the PCB. The SLICC uses an underfill between the IC device and the FR4 carrier to control thermal stresses on the flip chip connections. Overmolded pad array carriers (OMPAC) use conventional wire bonds to connect the IC device to a plastic carrier with solder balls used for connection to the PCB. The plastic carrier is overmolded similar to other plastic packages such as DIP and SOP packages.
U.S. application Ser. No. 08/754,869 filed on Nov. 11, 1996, the teaching of which is incorporated herein by reference describes a high density test probe for integrated circuit devices. The IC wafer probe structure is similar to the compliant interface used for the pluggable chip scale package. The probe structure described in this docket uses short metal wires that are bonded on one end to the fan out wiring on a rigid substrate. The wires are encased in a compliant polymer material to allow the probes to compress under pressure against the integrated circuit device. The wire probes must be sufficiently long and formed at an angle to prevent permanent deformation during compression against the integrated circuit device. Although the compliant interface uses a similar integrated wire conductor, ball shaped contact, and elastomeric material surrounding the wires, the IC packaging requirements includes additional features that are unique to the pluggable chip scale package.
There are a number of commercially available connectors for BGA and LGA packages. These connectors are separate components from the IC package and use a variety of contact geometries and materials. Conventional metal spring contacts can be formed in a variety of shapes to produce an interposer contact. Conventional metal spring contacts are typically contained in a plastic housing that have individual openings for each of the contact springs and corresponds to the array of pads or bumps on the IC package and the pads on the PCB. Elastomeric connectors are available that contain conductive wires or particles embedded in a sheet of elastomer material. The elastomer material acts as a spring element to press the conductive particles against the mating surfaces. As a separate component, an interposer connector must be aligned to both the PCB and the IC package.